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Photo size in pixels, cm and KB: what online forms actually mean

By the Fusion Office team · Updated

Quick answer

Pixels are the image's dimensions, centimetres are its print size (which needs a DPI to turn into pixels), and KB is how much storage the file takes. They're separate rules: a photo can be the right pixel size and still too many KB. Resize image sets the dimensions and caps the KB in one go.

The three measurements

  • Pixels (for example 200 × 230): how many dots wide and tall the image is. This is what the portal actually checks for dimensions.
  • Centimetres (for example 3.5 × 4.5 cm): how big the photo would print. It only becomes a pixel size once you choose a DPI: pixels = cm ÷ 2.54 × DPI.
  • KB (for example 20–50 KB): the file size on disk. It depends on the pixels and on how strongly the JPG is compressed.

How they affect each other

  • More pixels usually means more KB. Shrinking dimensions is the gentlest way to cut file size.
  • Stronger JPG compression lowers KB without changing pixels, but too much makes blocks and smudges.
  • Changing the DPI value stored in a file doesn't change its KB; changing the pixel count does.

So when a form says “3.5 × 4.5 cm, 20–50 KB”, you first get the right shape and pixel count, then compress until the file lands in the KB range. The full cm-to-pixel arithmetic is in How to resize a photo to 3.5 × 4.5 cm.

Meet all three at once

  1. 1Crop the photo to the right shape with Crop image.
  2. 2Open Resize image and enter the size in Pixels, or in Print size with the DPI the form gives.
  3. 3Turn on Limit the file size and enter the maximum KB.
  4. 4Save as JPG and check the file is above the form's minimum KB.

If only the KB is wrong, Compress image changes the file size without touching the dimensions; see How to compress a photo to 20, 50 or 100 KB.

Frequently asked questions

The form only gives KB. What size in pixels should I use?

Around 300–600 pixels on the long side suits most form photos and keeps small KB targets easy to hit without blurring.

Why is my 200 × 230 photo still too big?

It's saved at very high quality or as PNG. Save it as JPG with a KB limit.